Method for a thorough desulfurizing of molten metal and in particular of liquid pig iron



United States Patent METHOD FOR A rnonoucn DESULFURIZlNG or MOLTEN METAL AND IN PARTICULAR or LIQUID PIG IRON Lucien Wahl, Saint-Germain-en-Laye, France, assignor to Institut de Recherches de la Siderurgie, Saint-Germain-en-Laye, France, a French professional institution Application May 10, 1956, Serial No. 584,082

Claims priority, application France May 14, 1955 6 Claims. (Cl. 75-51) It is known to desulfurize molten metal and in particular pig iron by means of different methods. In that respect the co-pending patent application Ser. No. 567,032 of February 21, 1956, is relative to a method using a container provided with tuyeres in its lining.

The method described in said patent application offers unquestionable advantages in speed and economy, but it is possible to effect certain improvements whereby it may be rendered more economical and more eflicient.

The main object of the invention is to provide a method of desulfurizing molten metals based on the same principle, that is a method comprising blowing a desulfurizing medium in a highly subdivided state suspended in a very small amount of non-oxidizing gas through the tuyeres of a container, but using two desulfurizing agents consecutively.

Another object of the invention is to provide a method as above stated, the second desulfurizing agent being so eflicient that nodular iron is obtained.

Other objects and advantages of the invention will be apparent during the course of the following description, reference being made to the accompanying drawing, the single figure of which is a diagrammatic showing of a plant intended for the execution of my invention inside a converter as a container for the reaction media.

As illustrated, the plant includes chiefly an apparatus 1 in the shape of a steelwork converter, said converter being adapted to be tilted round its axis 2 through the agency of an electric motor which is not illustrated, the plant including further a powder-distributor 3 designed in a known manner and feeding uniform outputs of highly concentrated suspensions of a very fine powder, a group of bottles 4 containing compressed nitrogen or any other inert or reducing gas, a pipe 5 feeding the nitrogen with the subsequent admixture of the desulfurizing reagent to the tuyeres 6 extending through the bottom of the small converter and lastly a control board 7 for checkingand adjusting operation.

This control board shown very diagrammatically at 7, carries the manometers, the flowmeters and the different valves through which it is possible to control and adjust the flow of the fluid carrying the operative powder. I have illustrated in the drawing the fluid conveying channels 8 connecting the valves and measuring instruments on the control board with the corresponding elements of the blowing system. I have shown at 9 the scale which allows adjusting as required the opening of the powder distributor device. Lastly I have illustrated at 10 the pressure reducing and measuring valve for the battery of gas-containing bottles of which only two have been illus trated at 4 on the drawing.

To the refractory wall 11 of the converter is secured the converter bottom at 12 provided with tuyeres 6 forming tubular connections with threaded outer ends 13, which ends carry through the agency of fastening collars 14 the lime distributing tubes 15. The substances to be projected into the converter are fed through a yielding pipe 16 connected with the tube 5 at one end and through the threaded connection 17 at the other end with the lime distributing tubes In order to introduce the second desulfurizing agent the installation comprises a small tank 18 containing powdered magnesium, for instance. This small tank is provided at its lower part with a stop-cock 19 and is situated as indicated in the accompanying drawing, nitrogen under pressure being employed to stimulate the flow of powder.

Many other types of apparatuses may be used for executing my invention. Thus, instead of a converter, we may resort to a tilting ladle which serves both for the transportation of liquid pig iron and for the execution of the desulfurizing operation. During the transportation of the liquid pig iron, the tuyeres in the ladle project above the level of the molten metal so that they may be brought through a tilting movement of the ladle round its axis underneath the level of the bath during the desulfurizing operation.

- powder, said suspensions being very concentrated so as to damp the cooling action of the carrier-gas and to reduce the duration of the operation; the suspensions formed are then blown consecutively into the metal bath through the openings provided in the bottom or along the wall of the metallurgical container carrying the molten metal to be treated, said openings registering with the inside of the bath underneath the level of the latter in the container.

The desulfurizing agents used should be of a very fine granulometry such for instance that they may pass entirely through a 20 mesh Tyler sieve. It is possible to use advantageously as a first agent pulverulent lime or more generally an oxide or a compound of alkaline or earth alkaline metal or magnesia or a mixture of such oxides or compounds.

It is also possible to resort to one or more of the precedingly mentioned desulfurizing agents with the addition of pulverulent reducing substances.

The first desulfurizing agent may also act as a reducer in which case it is constituted e.g. by calcium carbide or calcium cyanamide.

After the desulfurizing has been performed as far as a predetermined sulfur content by means of said first desulfurizing agent, which is generally inexpensive, a very energetic desulfurizing mixture, generally of a very expensive nature, is bubbled or blown through, to eliminate the remaining sulfur and achieve extremely low contents less than 0.004%.

As to the carrier gas it is also possible to use various gases for this purpose. Although we prefer using nitrogen, it is possible to incorporate into the carrier gas a reduc ing gas such for instance as hydrogen or carbon monoxide or hydrocarbons.

The following example is given by no means in a limiting sense but only to show a means for executing my invention together with the result which may be expected.

The small converter 1 provided with a basic lining and tuyeres associated therewith is heated up to 1000 C. and laid horizontally. A ladle containing about 300 kgs. of liquid pig iron previously melted in a small induction furnace is then brought near the converter so that the metal may be poured into the latter.

Immediately after the liquid pig iron has been poured into the converter, a sample is removed and shows sulfur contents of 0.080%, the temperature of the liquid pig iron being equal to 1300 C. The nitrogen is then introduced and a few seconds. afterwards, the powder distributor is adjusted so as to produce the desired flow. The desulfurizing agent used in said example is constituted by crushed commercial burnt lime adapted to pass through a 20 mesh Tyler sieve. It is a well known fact that lime when crushed assumes for its major part the form of an extremely fine powder. The appearance of a White cloud at the mouth of the converter is a signal meaning that the converter is then to be raised into its vertical position. The blowing is then performed normally without any substantial projection out of the converter. A flame appears at the mouth of the converter and this flame shrinks gradually during the operation together with the small cloud of lime; in the case considered the output of lime is equal to about 3 kgs. per minute. After the blowing has lasted about 3 minutes, the converter is returned into its horizontal position while its lime and nitrogen circuits are cut off in succession. A sample removed 20 seconds after the converter has been returned into its horizontal position shows sulfur contents of 0.006%, which corresponds to a rate of desulfurization equal to 93% The flow of nitrogen is then resumed and the converter raised whilst at the same time the stop-cock 19 is opened. 200 grams of lime, containing of its weight in powdered magnesium, substance which was enclosed in the small tank 18, are bubbled through the iron through the tuyeres at the base and react instantly with the metal bath, reducing the sulphur content to less than 0.004%, a value which cannot be determined by current methods of analytical chemistry.

This operation lasts only a few seconds, whereupon the iron is then tapped and it is found that it contains 100% spheroidal graphite.

It should be observed at this point that, in normal industrial practice, spheroidal graphite iron is obtained by introducing into the molten metal certain metals or alloys, most often magnesium or alloys of magnesium. Unfortunately, such products are very costly and their action feeble and often unreliable, thus increasing the overall cost of the operation. In consequence of the above therefore one of the novel characteristics of the invention consists in injecting the new nodulizing product, which is generally expensive, only in order to eliminate the very slight remaining quantities of sulphur which have been obtained by previous use of the method to which the co-pending patent application relates. By this means an important economy is achieved.

It is manifest that, without departing from the framework of the invention, modifications and improvements in detail can be envisaged, apart from the possible use of equivalent means.

What I claim is:

1. In a method for speedily and efiiciently desulfurizing a bath of molten metal, chiefly liquid pig iron, down to a product which after tapping contains spheroidal graphite, said pig iron being poured for forming a bath in a container provided with tuyeres in its lining, said container being adapted to be tilted and wherein no previous oxidizing reactions were performed, the tuyeres being above said bath when not in an operative position, the steps of suspending a first desulfurizing medium in a highly subdivided state in a very small amount of nonoxidizing gas, tilting said container to make the tuyeres register with the body of said bath, blowing said first desulfurizing medium with said non-oxidizing gas through the tuyeres, andblowing a second and more energetic desulfurizing mixture with said non-oxidizing gas to eliminate the remaining sulfur down to extremely low contents less than 0.004%.

2. The method of claim 1 wherein said first desulfurizing medium is pulverulent lime.

3. The method of claim 1 wherein said second desulfurizing medium is magnesium.

4. The method of claim 1 wherein said second desulfurizing medium is a mixture of lime and magnesium.

5. The method of claim 1 wherein said non-oxidizing gas is nitrogen.

6. In a method for speedily and efliciently desulfurizing a bath of molten metal, chiefly pig iron, poured for forming a bath in a container provided with tuyeres in its lining, said container being adapted to be tilted and wherein no previous oxidizing reactions were performed, the tuyeres being above said bath when not in an operative position, the steps of suspending pulverulent lime in a highly subdivided state capable of passing through a 20 mesh Tyler sieve in a very small amount of nitrogen, tilting said container to make the tuyeres register with the body of said bath, blowing said pulverulent lime with nitrogen through the tuyeres, stopping the blowing, and blowing a mixture of lime and magnesium with nitrogen to eliminate the remaining sulfur down to extremely low contents less than 0.004%.

References Cited in the file of this patent UNITED STATES PATENTS 239,621 Pirath Apr. 5, 1881 387,952 Fronheiser Aug. 14, 1888 2,290,961 Huer July 28, 1942 2,577,764 Hulme Dec. 11, 1951 2,622,977 Kalling et a1. Dec. 23, 1952 2,652,324 Hignett Sept. 15, 1953 FOREIGN PATENTS 622,419 Great Britain May 2, 1949 

6. IN A METHOD FOR SPEEDILY AND EFFICIENTLY DESULFURIZING A BATH OF MOLTEN METAL, CHIEFLY PIG IRON, POURED FOR FORMING ABATH IN A CONTAINER PROVIDED WITH TUYERES IN ITS LINING, SAID CONTAINER BEING ADAPTED TO BE TILTED AND WHEREIN NO PREVIOUS OXIDIZING REACTIONS WERE PERFORMED THE TUYERES BEING ABOVE SAID BATH WHEN NOT IN AN OPERATIVE POSITION, THE STEPS OF SUSPENDING PULVERULENT LIME IN A HIGH SUBDIVIDED STATE CAPABLE OF PASSING THROUGH A 20 MESH TYLER SIEVE IN A VERY SMALL AMOUNT OF NITROGEN, TILTING SAID CONTAINER TO MAKE THE TUYERES REGISTER WITH THE BODY OF SAID BATH, BLOWING SAID PULVERULENT LIME WITH 